This invention pertains to the field of electrical circuit protectors, and more particularly to the field of dual element cylindrical line fuses.
Electrical circuit protectors, or fuses, are used in electric circuits to protect equipment from electric overload conditions. The fuse is placed in a power supply circuit adjacent the equipment to be protected, and will open the circuit to interrupt the flow of power to the equipment under prescribed current overload conditions. Such overloads could occur from a surge in line amperage caused by many factors, including a short circuit condition in the equipment to be protected.
One common protector is the cylindrical dual element line fuse. Prior art line fuses generally include a thin-walled, hollow insulating sleeve or tube having conductive end caps disposed on the ends of the tube and a conductive element or series of elements disposed within the tube between the conductive end caps. The conductive ends fit into clip terminals in the power supply circuit to permit the fuse to complete an electric circuit. During normal circuit conditions, current flows through the fuse without interruption. However, during overload conditions, the conductor fusing element forming a link in the conductive path overheats and melts, causing the element to sever, thereby preventing current flow through the fuse and opening the circuit.
The typical dual element fuse may have two separate fusing elements for initiation of fuse opening. One element, a strip and spring assembly, responds to long-term slight-to-moderate overcurrent conditions. The second element, a short circuit strip, opens the fuse in response to short circuit conditions. These separate fuse elements are connected in series to complete a conductive electrical path through the fuse body. The strip and spring assembly is comprised of a heater strip and a spring loaded in tension. One end of the heater strip of the strip and spring assembly is placed in electrical contact with one end cap. The short circuit strip is placed in electrical contact with the other end cap. The heater strip and the short circuit strip are threaded through the sleeve, and are connected to opposed ends of a conductive heater coil located on an insulator assembly housed near the center of the fuse. The insulator assembly divides the sleeve into two cavities. The cavity within which the short circuit strip is located is loaded with electric arc quenching filler such as sand, and the cavity in which the heater strip is located has air. Each cavity is substantially sealed to the environment exterior the fuse. The insulator assembly has a hollow aperture therethrough which forms a channel through which the arc quenching filler can travel between the two cavities. If too much of the filler material migrates through the channel, the arc quenching property of the silicates surrounding the short circuit strip is diminished, and the heat sink mass adjacent the heater coil is increased, thereby diminishing the effectiveness and safety of the fuse. To prevent migration, a washer is placed over the aperture in the assembly insulator.
The typical insulator assembly includes a cylinder having a separate insulator flange disposed on one end. The insulator flange includes a circular aperture at its center which is coaxially aligned with the bore of the cylinder. A coil of heat generating resistance wire is disposed about the exterior of the cylinder. A metal conductive hollow cylindrical eyelet, having an integral metal flange at one end, is disposed within the cylinder with the metal flange abutting the end of the cylinder opposite the cylinder-insulator flange interface. The end of the eyelet protrudes through the cylinder and aperture in the flange and the edge of the end of the eyelet is then roll crimped, retaining the cylinder and flange in contact. This maintains the coil in place between the insulator flange and metal flange. One end of the coil is bent, by hand using a pair of pliers, over the outer edge of the metal flange prior to assembly of the eyelet into the cylinder. After assembly, the opposite end of the coil protrudes through the insulator flange. The strip and spring assembly are connected to the metal flange by a solder like fusing alloy. The fusing alloy has a specific preselected melting point capable of being generated by the buildup of heat within the heater coil caused by the movement of long-term excess current through the fuse. The ends of the short circuit strip and strip and spring assembly are oppositely disposed to the conductive end caps. Thus, an electrical path is created, from one conductive cap, through the short circuit strip, coil, strip and spring assembly and into the opposed conductive cap.
The two fusing elements are held in place in the tube by the end caps. The heater strip portion of the strip and spring assembly is held by a retainer placed between one end cap and the end of the tube. The retainer holds the heater strip in conductive contact with the end cap. The spring is held in tension by a hanger placed across the end of the insulating tube. The short circuit strip is retained in electrically conductive contact with the other end cap through a circular slotted conductive washer housed in the end cap. The end of the short circuit strip is passed through the slot in the washer, and the washer is retained between the end cap and sleeve end. The heater strip in the strip and spring assembly and the short circuit strip are normally made from the same materials. However, the heater strip has a greater cross-sectional area than the short circuit strip.
The insulator assembly is located in the tube by a series of notches punched into the outer periphery of the tube thereby creating protrusions on the inner surface of the tube. The insulator flange engages these protrusions and is held against them by the tension of the spring.
In operation, normal electrical current, i.e., fuse-rated current, flows through the fusing elements. However, the fuse will sever and open under two conditions: a short circuit or a long-term overload condition. In a short circuit condition, the electric current passing through the fuse is substantially instantaneously increased, causing the short circuit strip to melt and consequently sever, thereby opening the circuit. However, modest overloads, on the order of 500 percent of rated capacity for less than ten seconds, will not ordinarily cause the short circuit strip to melt and sever. To address this problem, the heater strip in the strip and spring assembly is attached to the metal eyelet flange with a low melting point fusing alloy. The fuse is designed so that during overload conditions heat will be generated in the resistance wire coil, thereby increasing the temperature at the heater strip-metal flange junction. After a certain period of time, the heat will be sufficient to raise the solder temperature to its melting point. At this point, the spring will pull the heater strip away from the flange, thereby opening the fuse.
The above prior art construction has several deficiencies. First, the insulator and cylindrical eyelet crimped coaxially therein are hollow, and therefore the arc quenching filler material in one portion of the fuse will pass into the eyelet, increasing the heat sink mass adjacent the coil and thereby affecting the fuse rating. To overcome this filler transfer problem, the prior art requires that a washer over the end of the flange between the flange and filler material. Second, the prior art requires the additional operation of bending the coil during manufacture to assemble the coil to the metal flange. Finally, the two-piece insulator comprised of the cylinder and insulator flange is difficult to assemble.